1. Field of the Invention
The present invention relates generally to distortion suppression techniques, and more particularly to the elimination of interference between juxtaposed radio transceivers.
2. Description of the Prior Art
In order to accommodate spacing restrictions of multiple receive/transmit radio systems, it is preferable to collocate transceivers on a common platform. While the collocation of transceivers yields a radio system that is relatively small, the placement of transceivers in close proximity to one another produces cosite interference between transceivers. Cosite interference is caused by radiated and conducted interactions of transmitters and receivers. When the interference is severe, in-bound radio communications that are to be provided to a receiver can be totally disrupted and unrecognizable.
Unwanted (i.e., interfering) signals manifest themselves in several ways. Interference can cause a reduction in the sensitivity of a receiver (receiver desensitization), masking of a desired signal, tracking of an undesired interfering signal and loss of the desired signal, and processing of the unwanted interfering signal instead of the desired signal. Each of these manifestations of interference limits the communication capabilities of the radio system afflicted by this problem. The effects of interference can be some combination of the absence of usable output from a receiver, false signals from a receiver, and malfunction of a device which is operated by the receiver. During emergency situations, the loss and corruption of the desired signal can be critical.
Unwanted signal interference is generally caused by modulation of signals provided to the receiver by the carrier waves, or by the wideband noise, generated by collocated transmitters. Unwanted signal interference also occurs when frequency-hopping transmitters are transmitting signals at frequencies that are substantially close to the frequency of the desired receiver signal (i.e., co-channel operation). Unwanted signal interference is also caused by "pseudo white-noise" generated by transmitters over a wide band of frequencies on either side of the transmitter's operating frequency. It is often found in collocated transceiver systems that this "pseudo white-noise" reaches unacceptable levels within the operating band of adjacent receivers. Unwanted signal interference is also attributed to signals (i.e., spurious emissions) generated by transmitters at odd harmonics of the fundamental frequency of the transmitter output signal. This is caused by the non-linear transfer characteristics of transmitters. Moreover, signals received by an antenna can be corrupted when multiple antennas are used to transmit and receive RF (radio frequency) electronic signals. This type of interference is typically referred to as antenna pattern distortion. Antenna pattern distortion can occur even when all but one of the antennas of the multiple antenna arrangement is operating.
In order to substantially reduce and eliminate the undesired interfering signals while maintaining the spacial benefits afforded by proximately locating transceivers, especially frequency-hopping transceivers, several signal processing techniques have been proposed. These techniques include agile filtering, agile filtering with multicoupling and interference cancellation.
Agile (frequency hopping) filtering includes coupling a frequency adjustable filter to the input of a corresponding receiver wherein the filter has been tuned to the operating frequency of the receiver. The frequency tunable filter is preferably a bandpass filter which removes unwanted signals being transmitted outside the filter bandwidth. As a result of the agile filtering technique, the RF sensitivity and selectivity of the receiver to which the filter is coupled is increased. In addition, it follows that the jamming bandwidth of the collocated transmitters is substantially reduced. However, if the interfering signals generated by the transmitter fall within the frequency band of the receiver, since the filter is tuned to the operating frequency of the receiver, the filter will not remove the interfering signal and the desired signal will remain distorted.
Use of the agile filtering technique is dependent upon priority management of frequency plans. Priority management involves assigning a rank to each transceiver and monitoring their current operating frequency. If collocated transmitters and receivers are assigned substantially similar operating frequencies, the transmitter or receiver that has previously been judged to be subordinate to the primary transceiver is turned off until reassignment of operating frequencies occurs. Since transceiver operation is temporarily suspended, information that was being either transmitted to, or received by, the subordinate transceiver is lost. During emergency situations, the loss of this data can be critical.
The second technique, agile filtering with multicoupling, is similar to the above-described technique. However, the second scheme includes coupling all transceivers, along with their respective agile filters, to a single antenna in order to prevent antenna radiation pattern deformation. Following the first technique, the frequency tunable filters of the second scheme also operate in accordance with priority management. Therefore, this technique suffers from the same drawbacks as described above because transceivers are temporarily shut-off resulting in a loss of information.
Provided that priority management is practiced, each of the above two techniques provide improvement over the signal transmission/reception exhibited by systems which do not employ these schemes. However, when the frequency band for allocation is limited, and there are a relatively large number of transceivers, implementation of priority management results in the loss of substantial amounts of data because of frequent conflicts in frequency allocation.
A third technique that has been utilized to remove unwanted interfering signals from a desired signal is interference cancellation. Interference cancellation involves sampling the transmitter output signal in order to eliminate from the desired signal, any interfering signal having a frequency proximate to the transmitter carrier frequency. When the signal noise and spurious sidebands generated by the interfering transmitter are strong, interference cancellation is inadequate. In addition, since the frequency of the interfering signal may arbitrarily vary as compared to the carrier frequency of the signal being transmitted by the interfering transmitter, interference cancellation does not provide sufficient cancellation of wideband noise including transmitter "pseudo white-noise" and spurious emissions.
An alternative interference cancellation scheme involves the cancellation of substantially all interfering signals outside the receiver's operating frequency band. As a result, the receiver is substantially protected from both the interfering carrier frequency of the transmitter signal and its associated sidelobes. While the alternative interference cancellation scheme does eliminate some interfering signals generated by the transmitters, it does not eliminate interfering signals that have substantially the same frequency as the received signal. In addition, this alternative interference cancellation technique is difficult to implement due to its complexity and due to the requirement that it operate over a high dynamic frequency range of signals. This usually requires multiple cancellation loops which are relatively difficult to implement.